Vitamin D status, defined by blood
25OHD concentration at baseline, is
not an inclusion criterion because 1) in
small trials, vitamin D appeared to benefit
those with prediabetes irrespective
of baseline 25OHD concentration
(10,11); 2) the definition of optimal vitamin
D status is controversial, and no consensus
exists on optimal blood 25OHD
level (15–19); 3) 25OHD concentration
varies by season and race (15,20); 4)
low 25OHD concentration is common in
the U.S. adult population, especially
among overweight and obese people
(15,21); 5) the study is designed to be
as inclusive as possible to ensure that
results are generalizable to clinical practice;
6) baseline 25OHD is a potential effect
modifier that will be tested in
subgroup analyses; and 7) screening
with blood 25OHD would be cumbersome
and expensive. Blood for 25OHD
concentration is collected at yearly intervals
and will be measured after the study
is completed to assess the efficacy of the
supplementation at raising 25OHD concentration
and to evaluate for heterogeneity
of treatment effect.